1. Field of the Invention
The present invention relates to a thermally-assisted magnetic recoding head and a magnetic recording apparatus.
2. Description of the Related Art
In recent years, a thermally assisted magnetic recording system has been proposed as a recording system for attaining a recording density of 1 Tb/in2 or more (refer to H. Saga, H. Nemoto, H. Sukeda, and M. Takahashi, Jpn. J. Appl. Phys. 38, Part 1, pp 1839 (1999)). In the existent magnetic recording apparatus, when the recording density exceeds 1 Tb/in2, this results in a problem of erasing recorded information by thermal fluctuation. In order to prevent this, it is necessary to increase the coercivity of a magnetic recoding medium. However, since the magnitude of magnetic fields that can be generated from a recording head is limited, recording bits cannot be formed to the medium if the coercivity is increased excessively. For overcoming the problem, in the thermally-assisted recoding system, a medium is heated by light to lower the coercivity at the instance of recording. This enables recording to a high coercivity medium to attain a recording density of 1 Tb/in2 or more.
In the thermally-assisted magnetic recoding system, it is necessary to heat the vicinity of a magnetic pole for applying a magnetic field by light. Therefore, for instance, a waveguide is formed to the magnetic pole and light of a semiconductor laser as a light source is introduced near the top end of the magnetic pole. In this case, a semiconductor laser is mounted on a flying slider or placed at the base of a suspension, and light is guided therefrom to a flying slider using a waveguide such as an optical fiber (refer to Kenji Kato, et al., Jpn. J. Appl. Phys. Vol. 42, pp 5102-5106 (2003)).
Further, a semiconductor laser may be placed on a suspension, and light thereof may be propagated as free wave propagation light and the light may be coupled to a waveguide by using a grating coupler (refer to Edward Gage et al., Technical Digest of Magneto-Optical Recording Internal Symposium 2006, p 2 (2006)).
In the thermally assisted magnetic recording apparatus, the semiconductor laser for irradiation of light is disposed on a suspension or an arm situated at the base thereof, or on a flying slider. In a case of placing the semiconductor laser on the suspension or the arm, light emitting from the semiconductor laser is guided through a waveguide or as free wave propagation light into the slider. In a case of guiding the light through the waveguide, the intensity of the light transmitting through the waveguide changes due to external disturbances such as vibrations or temperature fluctuation applied to the waveguide connecting the semiconductor laser and the slider. As a result, the intensity of light reaching the slider is changed. Further, in a case of guiding the light as free wave propagation light as far as the slider and coupling the propagated light by way of a grating coupler with the waveguide formed on the side of the magnetic pole, the ratio that the propagated light couples the waveguide formed to the side of the magnetic pole (coupling efficiency) depends on an incident angle of light that is incident to the grating. Accordingly, when the slider or the suspension vibrates, since the direction of the light incident to the grating changes, the coupling efficiency to the waveguide changes and, as a result, the intensity of the light transmitting through the waveguide formed on the side of the magnetic pole fluctuates. Further, in a case of using the grating coupler, the coupling efficiency to the waveguide also depends on the wavelength. If the temperature fluctuates, since the wavelength of light of the semiconductor laser fluctuates, the coupling efficiency changes. As a result, the intensity of the light transmitting through the waveguide formed on the side of the magnetic pole changes.
In a case of placing the semiconductor laser on the flying slider, for coupling the emission light from the semiconductor laser to the waveguide formed on the side of the magnetic pole, the semiconductor laser is disposed such that the emission end of the semiconductor laser is in contact with the incident end of the waveguide. Alternatively, the emission light from the semiconductor laser is condensed by a micro lens placed on the slider and the incident end of the waveguide is positioned to the focal point thereof thereby introducing the light into the waveguide. In this case, the amount of the light transmitting through the waveguide may possibly change by the following factors:    (1) Due to the degradation of an adhesive or solder that fixes the semiconductor laser or the micro lens, the position for the semiconductor laser or the micro lens is displaced during long time use to change the coupling efficiency between the waveguide and the incident light.    (2) Thermal deformation is caused to the slider or the optical element due to the heat generated from the semiconductor laser or the heat generated in the drive, to change the coupling efficiency between the waveguide and the incident light.    (3) For increasing the coupling efficiency between the waveguide and the incident light, the diameter for the light distribution in the waveguide is preferably made about equal with the spot diameter of the incident light. The diameter of the mode field means the width of the intensity distribution of the light in the waveguide. Usually, the diameter of the light spot at the emission end of the semiconductor laser is several μm. Even when this is condensed by a lens, it can only be restricted to about 1 to 2 μm due to the limit of diffraction. Accordingly, it is preferred to increase diameter of the mode field to about 1 to 2 μm. On the other hand, in the thermally assisted magnetic recording, it is preferred that the diameter of the light spot is decreased about to the same extent as the recording bit. If the diameter of the light spot is larger than the recording bit, an adjacent bit is heated to erase the recording bit thereof. For overcoming the problem of erasing the adjacent bit, a micro light spot is generated by utilizing an optical near-field generator. For example, an optical near-field generator such as a metal scatterer of a trigonal shape is disposed at the emission end of the waveguide in the slider (refer to T. Matsumoto, et al., Optics Letter, Vol. 31, P 259, (2006)). In this case, for enhancing the efficiency of generating the near-field light, it is preferred to make the spot diameter of light incident to the optical near-field generator as small as possible. That is, it is preferred to make the diameter of the mode field in the waveguide as small as possible. As a method of satisfying the requirement described above, there may be considered a method of making the width of the waveguide larger at the inlet of the waveguide, which is made gradually smaller as it approaches the optical near-field generator. In this case, since the width is large at the inlet of the waveguide, there may be a possibility that higher order propagation modes as well as fundamental mode is also excited. If the higher order modes are excited, the higher order modes and the fundamental mode cause interference in the waveguide. Then, distribution of the light intensity in the waveguide changes due to external disturbance such as temperature. As a result, the intensity of the light transmitting through a narrowed portion of the waveguide fluctuates.
While descriptions have been made to the fluctuation of the light intensity due to the external disturbances such as temperature change or vibrations, the light intensity changes also by the aging degradation of the semiconductor laser. If the intensity of the light incident to the surface of the medium fluctuates due to the external disturbances such as temperature change or vibrations, or the aging degradation of the semiconductor laser, the heating temperature of the medium changes. As a result, the recording condition changes on every time and stable recording is no longer possible (bit error rate increase).